I found this articles method of looking at the risk-premium of stocks to bonds, and small-caps to total-market, pretty interesting. Uses a lotto-share approach to explain how indexing increases your likelihood of getting the market return, since the majority of stocks fail, and a select few have massive returns. This is all fairly intuitive, but I thought it was explained in a nice concise way.

It goes on to show a plot of the % of funds in index funds relative to Cape-Shiller PE10, and claims this justifies the higher valuations of the 21st century.